1. Field of the Invention
The present invention relates generally to wafer cleaning apparatus, and particularly to a wafer cleaning apparatus removing contaminant particles sticking onto a wafer.
2. Description of the Background Art
When a film is formed on a wafer with a CVD method or a sputter method, contaminant particles stick onto the surface thereof. Residue of a resist also sometimes sticks onto the surface. High pressure jet spray cleaning, ultrasonic cleaning and ice scrubber cleaning have been proposed for removing such contaminant.
FIG. 15 is a schematic diagram showing the conception of an apparatus implementing a conventional cleaning method referred to as high pressure jet spray cleaning.
The apparatus includes a liquid pressurizer 3, a spray nozzle 4, and a stage 2 supporting and rotating a wafer 1. In this cleaning method, liquid such as pure water compressed by liquid pressurizer 3 is initially spurted continuously at high speed from spray nozzle 4 toward the surface of wafer 1. The spurted liquid collides against the surface of wafer 1, so as to clean the surface, removing contaminant particles sticking thereonto. The whole surface of wafer 1 is cleaned by rotating wafer 1 with rotation of stage 2 and moving spray nozzle 4.
Referring to FIG. 16, the spurted liquid from spray nozzle 4, being in the form of a column 20, collides against the surface of wafer 1.
In this method, a large amount of liquid such as pure water collides against the surface of wafer 1 at high speed, which gives rise to a problem of generating a static charge on the surface of wafer 1, thereby damaging a device thereon. Although it may be considered, for reducing this damage, to reduce resistivity of pure water by mixing a CO.sub.2 gas and the like with the pure water, thereby reducing a static charge generated on the surface of wafer 1, it cannot fully prevent the damage. Another problem in this method is that fine foreign matter (a particle) not more than 1 .mu.m cannot be completely removed.
FIG. 17 is a schematic diagram showing a conventional cleaning method referred to as ultrasonic cleaning. The cleaning apparatus includes a stage 2 rotating a wafer 1, and a nozzle 5 applying high frequency energy of approximately 1.5 MHz to a liquid such as pure water and discharging the same. A wafer is cleaned by horizontally vibrating pure water at a high frequency with nozzle 5, and discharging the same toward wafer 1.
In this method, similarly to the high pressure jet spray cleaning, fine foreign matter (a particle) not more than 1 .mu.m cannot be completely removed. In addition, when a speed of rotation of stage 2 is raised for enhancing cleaning effect, the difference in cleaning effect on wafer 1 between the peripheral portion and the center thereof results in uneven cleaning of the surface of wafer 1. Still another problem of this method is that a miniaturized pattern of a device formed on the surface of wafer 1 might be destroyed because of a correlation which has been recognized between destruction of a device and cleaning effect. Although a frequency and a power of the applied energy, a speed of rotation of stage 2, and a distance between nozzle 5 and wafer 1 can be listed as parameters for controlling the destruction of a device and the cleaning effect, it is difficult to control these parameters because of their narrow control range.
FIG. 18 is a schematic diagram of an apparatus implementing a conventional cleaning method referred to as ice scrubber cleaning. The cleaning apparatus includes an ice making hopper 6 making ice particles. Ice making hopper 6 is provided with a supply spray 7 supplying ice making hopper 6 with pure water which is a liquid to be frozen. A spray nozzle 8 spurting ice particles toward wafer 1 is provided at the bottom of ice making hopper 6.
Description will now be made on the operation of the apparatus. A liquified gas, such as liquid nitrogen, is supplied to ice making hopper 6. A liquid to be frozen, such as pure water, is finely sprayed into ice hopper 6 by supply spray 7. Ice particles of several to several tens of .mu.m made within ice making hopper 6 are sprayed from spray nozzle 8 of a gas ejector type toward wafer 1, thereby cleaning the surface of wafer 1. This cleaning method has higher cleaning effect, compared to the high pressure jet spray cleaning and the ultrasonic cleaning described above. However, detergency is limited, since spray nozzle 8 of the gas ejector type is employed, so that a spurting speed of ice particles determining the detergency cannot exceed the acoustic velocity. Moreover, since use of a large amount of liquid nitrogen for forming ice particles requires higher initial cost for an equipment for supplying liquid nitrogen, and also requires higher running cost. Furthermore, though the problem that a device might be destroyed, as described above, can be suppressed by controlling the spurting speed of ice particles, the controllable range of the spurting speed of ice particles is presently limited within 100 to 330 m/sec., considering problems on the apparatus. Because of such a narrow control range, destruction of a device cannot be fully suppressed.